An international group of space
scientists announced that they have made the first detections
of global short-term disturbances in the solar corona. These
disturbances result in global mass ejections from the Sun, which
in turn have varied, often massive, effects on Earth's magnetosphere.
Their findings were at the Spring Meeting of the American Geophysical
Union in Baltimore, MD on May 20. In contrast, previous observations
have revealed large outbursts called coronal mass ejections (CMEs)
which are propelled outward in a more single direction.

The group of American, German,
French, and British scientists, under the direction of Dr. Guenter
Brueckner of the Naval Research Laboratory (NRL), says that these
detections of short-term (hours to days) global coronal disturbances
may lead to a credible explanation of the previously known CMEs,
and the cause of the solar wind. The findings will also help
scientists formulate a model of the interaction of these events
with the Earth's magnetosphere and may lead to the ability to
forecast magnetospheric activity.

The group used observations from
NRL's Large Angle Spectro Coronagraph (LASCO) instrument on board
the ESA-NASA Solar and Heliospheric Observatory (SOHO)
satellite.

The new short-term global coronal
disturbances in the plane of the solar equator, are all around
the Sun not just in a single direction. Many of these global
coronal disturbances have been detected since the LASCO-SOHO
observations began in January 1996. "So they must be a very
common and probably very important process on the Sun,"
Dr. Brueckner said.

LASCO uses three coronagraphs
to observe the outer solar atmosphere from the solar limb to
a distance of 22 million km. Coronagraphs are special telescopes
that can image the faint corona in the presence of the glaring
light of the visible solar disk. These observations can only
be carried out from space because of the scattering of sunlight
in the Earth's atmosphere.

CMEs are clouds of hot (1-2 million
degrees) gasses ejected from the Sun at extremely high speeds
(from several hundreds to 2000 km per second). After acceleration
at the Sun, they travel through interplanetary space and reach
Earth in 2.5 to 5 days. When they reach the Earth, CMEs cause
disturbances in the magnetosphere, which trigger auroras, make
magnetic navigation at high latitudes difficult, and sometimes
cause current spikes in high-voltage power lines, resulting in
power outages and occasionally in destruction of power equipment.
They also can damage or destroy Earth-orbiting satellites.

LASCO began making observations
on December 29, 1995. On January 15, less than 3 weeks later,
and to the surprise of everybody involved in the project, the
#3 coronagraph observed a large CME. Then, only 4 days after
the #2 coronagraph was activated, LASCO's #3 and #2 coronagraphs
observed a second large event.

This second event, because it
was seen in the field of view of two coronagraphs, could be followed
from 1.1 million km above the solar surface out to 15 million
km. Figure 1 is a frame of the event during an eleven-hour observation.
Bright clouds are seen traveling outward in the equatorial plane
with speeds ranging from 90 km per second to 540 km per second
over both the east and the west limb of the Sun. The acceleration
takes place over a distance of 15 million km. The sectors above
the solar limb in which the bright clouds are seen seem to extend
over an angle
of 120 degrees in the equatorial plane. Although the instruments
cannot see material moving toward or away from the Earth-Sun
line, it is safe to assume that the CME extended all the way
around the Sun. A faint event can be seen in this picture above
the south pole of the Sun.

A thin magnetic current sheet
forms around the Sun during minimums of the solar cycle. The
acceleration of the global coronal disturbances seem to occur
in this sheet. The upper and lower boundary layers of this sheet
have opposite magnetic polarity. Consequently, the two boundary
layers attract and form a "lid," thus trapping hot
coronal material in the corona's outer layers. The amount of
material stored varies greatly with time. The upper panel of
Figure 2 shows the current sheet during a quiet period;
the lower panel shows the same area two days prior to the February
3 CME. The obvious increase in brightness prior to the CME indicates
that more material is stored before a global coronal disturbance
than during quiet coronal periods. It is assumed that the current
sheet will blow open when the pressure inside exceeds a certain
value. An instability will then accelerate and release the stored
hot coronal material as a global coronal disturbance.

The configuration of the solar
magnetic field during the minimum of the solar cycle, which the
Sun is approaching now, leads to the formation of this current
sheet. Weak polar fields are bending over toward the equator
to form the current sheet in the upper corona, which can be seen
in Figure 3 by following the polar boundaries of the hot
corona. The corona over the poles at this time of the solar cycle
is much cooler and therefore does not show up in this picture.

Many more global coronal disturbances
have been observed by LASCO since February 3. They seem to be
a regular feature of the corona during the minimum of solar activity.
This explains why the Sun was found to be so active during the
so-called "quiet" phase of its eleven year cycle.

Future LASCO research will address
the connection between global coronal disturbances and the Earth's
magnetic field. Observations of these new global coronal disturbances
at the Earth carried out from other satellites must be correlated
with those of LASCO before a credible picture of the Sun-Earth
connection can emerge.

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